1. Field of the Invention
The present invention relates to a mechanism for moving an X-ray source in order to obtain, on an X-ray film, panoramic radiographs of an object the shape of the dental arch. The mechanism is of the type which comprises a stationary frame, a movable support arm with the source of radiation and the film holder attached at its opposite ends on both sides of the object to be radiographed, and means for effecting the rotational movement of the support arm around the object.
2. Description of the Prior Art
In panoramatomographic radiography technique, an image of a layer of the object, the layer being of a certain depth, is formed on the film. The shape, location and thickness of this image layer can be affected by a suitable arrangement of the geometry relating to the forming of an image of the object. Normally in this technique, the X-ray source and the film move in relation to the patient, who is therefore immobile during the radiographing. A precise image of the desired spot of the object is formed on the film, if the velocity of the film is selected equal to the velocity of the projected image of the spot being radiographed on the film surface.
The following factors, among others, affect the results obtained in panoramatomographic radiography: the distance of the center of rotation of the X-ray source from the object being radiographed, the film and the X-ray source, the velocity of the film in relation to the velocities of the image spots of the object projected onto the film surface, the width of the X-ray beam on the film surface, the size of the focal spot of the X-ray source, the type of X-ray film used, the properties of the intensifying screens, and the quality and amount of radiation emitted by the source. The significance of these factors is explained in more detail in, for example, J. van Aken's article: Panoramic X-ray equipment, Reports of Councils and Bureaus/Joda, Vol. 86, May 1973. The most important equipment available on the market and the principles of operation of the same are also described in the said publication.
There are currently available several panoramic X-ray apparatus, developed by different manufacturers. One of the basic models is the Finnish Orthopantomograph, developed by T. Nieminen on the basis of Professor Y. Paatero's idea and manufactured by Palomex Oy. In the said apparatus, the patient is in a standing position during the radiographing; the X-ray source and the film holder move around the patient's head while the patient remains immobile. In the apparatus, the rotational movement of the X-ray source has three different fixed rotation centers; change from one rotation center to another is through a cycloidal movement.
In the apparatus marketed by S. S. White (U.S. Pat. No. 3,045,118), the patient is in a sitting position, the X-ray source has during the radiographing two different rotation centers in relation to the patient, but the X-ray source rotates around the same point throughout the procedure, in which case the change from one rotation center to another takes place by shifting the patient laterally over a suitable distance with the aid of the chair. This system has a disadvantage in that radiographing is not possible during the shifting, in which case the central area in the X-ray film must be left unexposed and thus the front teeth, which are in the middle of the dental arch, are not radiographed. In GE-3000 manufactured by General Electric (German (FRG) Patent Application No. 1 955 294), the movement of the rotation center of the source of X-ray radiation is based on a pair of elliptic gears and takes place along a curved path in a manner determined by the dimensioning of the gear pair.
Japanese Panex-"E" of Morita Corporation is a kind of application of the ellipsograph. Its principle of operation is disclosed in German (FRG) Patent Applications Nos. 2 057 135, 2 252 578 and 2 252 579. In this apparatus also, the rotation center of the X-ray source moves along a curved path.
There are also other similar apparatus on the market, but the mechanisms moving the X-ray source in these systems do not deviate substantially from the basic types mentioned above.
The point of departure in planning the mechanisms of all the apparatus currently on the market is some geometric curve close to the shape of the jaw bone, either an ellipse or a combination of two or three arcs of a circle, since thereby rather simple mechanisms can be constructed for moving the source of radiation.
However, if the planning is based on a shape as close to the jaw bone shape as possible and the optimal movement of the rotation center of the X-ray source in that case is determined thereafter, it is observed that the shape does not follow precisely any definite simple geometric shape. From this it follows that the movement used in current radiographic equipment is always to a certain extent an inaccurate approximation of the ideal.